1. Field of the Invention
The present invention relates to optical and electrical systems. More specifically, the present invention relates to sensors for laser guidance systems and laser spot trackers.
2. Description of the Related Art
Laser spot trackers are systems used for measuring the position of a laser spot, and are commonly used in missile guidance systems to guide a missile toward a designated target. In a semi-active laser guidance system, for example, a laser designator is pointed at a target, forming a laser spot on the target. A spot tracker or seeker on the missile senses the laser signal reflected from the target and measures the angle between the location of the laser spot and the missile heading. This angle measurement is then used to guide the missile toward the target.
Conventional laser guidance systems typically utilize non-eye safe designators (usually 1.06 μm lasers), thus putting friendly forces and non-combatants at risk of eye damage. Eye safety is of particular concern for smaller precision weapons that may be used in urban environments (for example, to target a single room in a building, a stationary or moving vehicle, or a small boat in a harbor) and also for spot trackers used in non-weaponry applications (such as imaging or reconnaissance).
New laser technology has been developed that can generate eye-safe laser energy at wavelengths that are not harmful to the eye (such as 1.54 μm). Detectors that are capable of measuring energy at these wavelengths, however, have been more difficult to develop. Most conventional detectors are made from silicon, which responds to wavelengths up to about 1 μm but will not respond to the longer, eye-safe wavelengths. Newer detector materials such as InGaAs or HgCdTe will respond to eye-safe laser wavelengths (as well as the more common 1.06 μm lasers), but detectors made from these materials must typically be very small due to the high capacitance of the material. A wide field of view therefore cannot be achieved without using either a large array of detectors or a scanning mechanism (e.g., a gimbal).
Gimbaled sensors can provide high angle measurement accuracy for a wide field of view, but are typically very expensive. Cost is often an important factor for laser spot trackers, particularly when used in missiles and small munitions. These applications usually prefer a body-fixed sensor without a gimbal.
Body-fixed sensors for laser guidance typically use a four-quadrant design, a simple and low cost approach that uses a single large detector that is divided into four quadrants. The missile is guided toward the laser spot by centering the received energy on the detector such that equal amounts of energy are detected in each quadrant. Four-quadrant detectors are typically made from conventional detector materials such as silicon. Eye-safe detectors made from InGaAs or HgCdTe would need to be arranged in a large array in order to achieve the same field of view as a conventional body-fixed sensor. Large detector arrays, however, are not compatible with existing four-quadrant designs.
Hence, a need exists in the art for an improved body-fixed sensor that can detect eye-safe lasers and which is compatible with existing laser guidance systems.